Nano LiMn2O4 with spherical morphology synthesized by a molten salt method as cathodes for lithium ion batteries

The compound, LiMn2O4 is synthesized by a one-pot molten salt method using NaCl-KCl (1 : 1) as the eutectic melt at various temperatures (T) from 700 to 850 degrees C and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Rietveld refinement, surface area and density methods. SEM showed that all the spinel phases LiMn2O4 consist of 1-5 mu m sized spherical particles, each of which is composed of similar to 50 nm nano-sized aggregates. TEM images show spherical particles with hollow type morphology when the synthesis T is above 800 degrees C. The cubic lattice parameter, 8.235 (+/- 0.002) angstrom did not vary much with the T in the range 700-850 degrees C, which is proven by the similar Mn3+ and Mn4+ amount through XPS results, whereas the surface area varied from 15.6 to 10.3 m(2) g(-1). The cyclic voltammograms showed the characteristic two-step redox peaks at 3.9/4.1 and 4.1/4.2 V vs. Li for all the compounds in agreement with literature reports. Galvanostatic cycling studies were carried out in the range, 3.5 to 4.3 V vs. Li showed that the LiMn2O4 prepared at 800 degrees C has the highest discharge capacity of 124 mAh g(-1) at second cycle at 0.25 C-rate, and it showed a capacity retention of 96% at 1 C, 2 C and 5 C-rates at the end of 50 cycles. Long-term cycling at 2 C-rate, up to 700 cycles showed a capacity retention of 81%. Thus, LiMn2O4 obtained at 800 degrees C with uniform hollow spherical particles shows the best electrochemical properties. Complementary electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) studies were carried out and the apparent Li-ion diffusion coefficients (DLi+) were calculated as a function of the applied voltage. The DLi+ values from GITT range from similar to 0.1 to 5 x 10(-10) cm(2) s(-1). The values decrease with an increase in the applied voltage and show two minima in good agreement with the available literature data.